Many electrical-energy generation systems are based on an electromechanical generator driven by an internal combustion engine. The engine normally runs on a petrochemical-based fuel, such as diesel fuel, gasoline, jet fuel, bunker fuel, and the like. In order to keep the system running for any extended period of time, fuel must be transported to, and stored at, the location where the system is operated. For generation systems used to power a reasonably sized, deployable facility, such as a temporary army base, scientific outpost, mobile hospital, etc., the amount of fuel necessary can be significant.
One way to reduce the amount of fuel that must be transported and stored on site is to use a “waste-to-energy” system to convert locally produced waste (e.g., kitchen waste, municipal solid waste, construction debris, organic waste, or other biomass—henceforth called biomass), into a fuel that can be used in the internal combustion engine. This not only mitigates the expense, complexity, and risks associated with fuel transportation and storage by augmenting the amount of available fuel for the engine, it also reduces waste-disposal overhead at the facility itself. As a result, waste-to-energy conversion systems have become a focus of attention of late.
A common waste-to-energy conversion system includes a gasifier and an internal combustion engine that drives an electric generator. The gasifier converts biomass into synthetic fuel through a process referred to as gasification. Gasification is a well-known process for converting carbon-based materials into gaseous fuels that contain carbon monoxide, hydrogen, carbon dioxide and methane. These vapor-phase fuels are referred to as producer gas or “syngas.” In a gasifier, raw material is reacted with a controlled amount of oxygen and/or steam at a high temperature, but with partial combustion. The resultant syngas is either provided directly to an engine designed for its use or, alternatively, converted into a more conventional liquid fuel, via a well-known conversion process (referred to as the Fischer-Tropsch process), for use by a conventional engine.
Unfortunately, during gasification of biomass into syngas, significant amounts of high-boiling-point aromatic hydrocarbon mixtures (e.g., tars, creosote, etc.) are typically generated and are contained in the output syngas stream. These tars are often problematic because once these vapor-phase tars reach exposed cooler surfaces, such as exit pipes, intake manifolds or particulate filters, they can condense and deposit on the surfaces as tars, creosote, etc. leading to blockages and clogged filters. In many applications, while a nuisance, the deposited materials can be removed via routine maintenance operations, such as chimney cleaning, and the like. For waste-to-energy conversion systems, however, the generated tars represent a much more significant problem. Internal combustion engines are extremely sensitive to tar buildup and complications that arise from its presence increase dramatically. The effect of tars on engine components, such as valves, fuel injectors, intake manifolds and fuel lines, can result in onerous maintenance and/or repair requirements.
As a result, great care is taken in prior art waste-to-energy conversion systems to avoid the impact of gasifier-generated tars on the operation of an internal combustion engine. These approaches have historically been directed toward the reduction of tar generation during the gasification phase and/or removal of generated tar from the gasifier output syngas stream.
Gasifiers most commonly used for the gasification of biomass are fixed-bed types that operate in either an updraft (counter flow) or downdraft (co-current) configuration. Updraft gasifiers generate much higher levels of tars in their output syngas than are generated by downdraft gasifiers. As a result updraft gasifiers have historically been avoided in favor of downdraft gasifiers for internal-combustion-engine-based applications in general, and waste-to-energy conversion systems, in particular.
In a downdraft gasifier, biomass is fed to the top of a burning mass and air is drawn down through the mass. The heat of combustion volatilizes much of the organic mass, which passes down to a char zone containing partially combusted material that had already been fed into the system. In the char zone, generated tars react with injected air, water, and carbon dioxide to convert the tars more completely to an output gas stream that includes carbon monoxide and hydrogen. Unfortunately, this configuration also results in more parasitic nitrogen, carbon dioxide and water leaving the gasifier. As a result, the energy content and energy potential of the output gas stream is significantly reduced because so much of the combustion has already occurred in the gasifier. When this gas is fed to the engine, work must be done to draw in and expel the added parasitic nitrogen and carbon dioxide.
After the syngas is generated, it is typically conditioned to remove the tars and other unwanted impurities via tar cracking, scrubbing, or plasma decomposition.
Tar cracking is used to break down the complex tar molecules into simpler light hydrocarbons, typically via the introduction of a catalyst. While tar cracking is an effective way to reduce the tar content in the output gas stream of a gasifier, it results in a lower overall energy mix of carbon monoxide, hydrogen, carbon dioxide, nitrogen and water, relative to the original tar-containing stream.
More commonly, generated tar is normally removed from the output gas stream using conventional scrubbers. Wet scrubbing is an effective process that condenses available tars out of the gas stream using a scrubbing medium, such as a scrubbing oil or process oil, or more typically, water. In addition, wet scrubbing often leads to the formation and accumulation of contaminated wastewater, which must then be treated to avoid polluting the local environment. Typically, however, the scrubbing medium becomes saturated over time and must be replaced or regenerated periodically as part of a routine maintenance program. This increases the overall cost and complexity of the waste-to-energy conversion system, as well as system operation.
Plasma systems have often been used in the prior art to degrade tar molecules in the output gas stream into their atomic constituents. These atomic constituents can then be recombined to form syngas constituents (i.e., carbon monoxide and hydrogen). In a plasma system, the gas stream is heated to very high temperatures (1100° C.) via an electric arc, which is energetic enough to degrade the tar molecules. Unfortunately, it requires a great deal of energy to generate the plasma, which reduces the overall energy-efficiency of the waste-to-energy conversion system.
A system that mitigates the deleterious effects of vapor-phase tars in a syngas stream on an internal combustion engine, while improving the overall energy balance of a waste-to-energy conversion system, would represent a significant improvement over the prior art.